Pulverizer mills are commonly used for crushing large coal pieces into small particles which are required for conventional coal fired boilers. A common type of pulverizer includes a flat or dished grinding table which is attached to and driven by a vertical spindle and three (3) large rollers or wheels which rotate around separate shafts as the table (or bowl) rotates with the vertical spindle. Large coal particles are introduced onto the table (or bowl) and are crushed as they are captured between the rollers and the table. An air stream (known as Primary Air Flow) passing upwardly around the bowl carries the crushed coal particles upward into the classifier through the classifier vanes and then out of the mill to the boiler through an outlet pipe (or pipes).
Occasionally large coal particles are swept up and out through the outlet pipe due to the high velocities of the Primary Air Flow inside the top of the classifier. This is an undesirable characteristic of all coal pulverizers. In order to minimize the amount of large coal particles which are swept out of the mill, a cone-shaped classifier has been used in all prior art designs for receiving partly crushed coal particles and for separating large particles which must be crushed further from the fine particles (which are desired). The interior surfaces of all prior art classifiers are smooth. Although the classifier is an integral part of all vertical spindle mill designs, it is not as effective as desired in many instances. Consequently, the grinding capacity of a mill can be limited because of the inherent inefficiencies of present classifier designs. Or conversely, the large amounts of unburned coal found in the ash of many typical boilers reduces the efficiency of said boiler and increases the operating costs of the user.
There has not heretofore been described a classifier system having the advantages provided by the present invention.